APOBEC3G (A3G) is a single-stranded DNA cytosine deaminase that can restrict HIV-1 infection by mutating HIV-1 genome. HIV-1 developed a counter defense mechanism by which virion infectivity factor (Vif) leads the degradation of A3G through ubiquitin-proteasome pathway. Our ultimate goal is to generate small compounds which inhibit the degradation of A3G. We have determined structures of two functional domains of A3G, including the VIf-binding domain and the catalytic domain. In addition, we have determined co-srystal structure of the A3G catalytic domain-ssDNA complex. By using NMR and Molecular Dynamics simulations, we revealed the mechanism by which APOBEC3 proteins exclude RNA as a substrate for deamination. We are working toward the structure determination of the A3G-Vif E3 ubiquitin ligase complex that will provide epitopes to be targeted by small compounds which inhibit formation of the complex. The A3G-Vif E3 ubiquitin ligase complex contains 6 proteins, and therefore it is challenging for any structural study. We are developing and optimizing biochemical and spectroscopic techniques to overcome the challenge.